Personal computers and servers and the like use a hierarchy of memory devices. There is lower-tier memory, which is inexpensive and provides high storage capacity, while memory higher up the hierarchy provides high-speed operation. The bottom tier generally consists of magnetic storage such as hard disks and magnetic tape. In addition to being non-volatile, magnetic storage is an inexpensive way of storing much larger quantities of information than solid-state devices such as semiconductor memory. However, semiconductor memory is much faster and can access stored data randomly, in contrast to the sequential access operation of magnetic storage devices. For these reasons, magnetic storage is generally used to store programs and archival information and the like, and, when required, this information is transferred to main system memory devices higher up in the hierarchy.
Main memory generally uses dynamic random access memory (DRAM) devices, which operate at much higher speeds than magnetic storage and, on a per-bit basis, are cheaper than faster semiconductor memory devices such as static random access memory (SRAM) devices.
Occupying the very top tier of the memory hierarchy is the internal cache memory of the system microprocessor unit (MPU). The internal cache is extremely high-speed memory connected to the MPU core via internal bus lines. The cache memory has a very small capacity. In some cases, secondary and even tertiary cache memory devices are used between the internal cache and main memory.
DRAM is used for main memory because it offers a good balance between speed and bit cost. Moreover, there are now some semiconductor memory devices that have a large capacity. In recent years, memory chips have been developed with capacities that exceed one gigabits. DRAM is volatile memory that loses stored data if its power supply is turned off. That makes DRAM unsuitable for the storage of programs and archival information. Also, even when the power supply is turned on, the device has to periodically perform refresh operations in order to retain stored data, so there are limits as to how much device electrical power consumption can be reduced, while yet a further problem is the complexity of the controls run under the controller.
Semiconductor flash memory is high capacity and non-volatile, but requires high current for writing and erasing data operations, and these operation times are long. These drawbacks make flash memory an unsuitable candidate for replacing DRAM in main memory applications. There are other non-volatile memory devices, such as magnetoresistive random access memory (MRAM) and ferroelectric random access memory (FRAM), but they cannot easily achieve the kind of storage capacities that are possible with DRAM.
Another type of semiconductor memory that is being looked to as a possible substitute for DRAM is phase change random access memory (PRAM), which uses phase change material to store data. In a PRAM device, the storage of data is based on the phase state of phase change material contained in the recording layer. Specifically, there is a big difference between the electrical resistivity of the material in the crystalline state and the electrical resistivity in the amorphous state, and that difference can be utilized to store data.
This phase change is effected by the phase change material being heated when a write current is applied. Data is read by applying a read current to the material and measuring the resistance. The read current is set at a level that is low enough not to cause a phase change. Thus, the phase does not change unless the material is heated to a high temperature, so data is retained even when the power supply is switched off.
In order to rewrite the data, it is necessary to pass a sufficient amount of current for causing a phase change. To change the phase from the crystalline phase to the amorphous phase, a particularly large amount of current is required as compared to changing the phase from the amorphous phase to the crystalline phase. Accordingly, if the cell transistor is miniaturized for enhancing a recording capacity, the current supply capability of the cell transistor decreases. As a result, a longer time is required to rewrite the data.
As a method for solving such a problem, Japanese Patent Application Laid Open No. 2005-71500 discloses a method in which two cell transistors connected in parallel are assigned to one non-volatile memory element. According to this method, an effective gate width is increased, thereby enhancing the current supply capability of the cell transistor.
However, when the cell transistor is further miniaturized, a sufficient amount of current cannot be secured in some cases even when the method described in Japanese Patent Application Laid Open No. 2005-71500 is employed. In order to achieve rewriting at a higher speed, the current supply capability of the cell transistor need to be further enhanced. Such challenge is particularly important for a so-called PRAM using the phase change material. At the same time, this challenge is equally important for other semiconductor memory devices that hold information depending on the presence or absence of a current that flows through a cell transistor, the magnitude of a current that flows through the cell transistor, or the like.
As another method for enhancing the current supply capability of the cell transistor, U.S. Pat. No. 6,862,214 discloses a method in which a transistor for short-circuiting adjacent memory cells is provided. When such a transistor is provided, however, the control becomes complicated since, for example, a bit line needs to be set to a temporary floating state.